1. Field of the Invention
The present invention relates to an apparatus for polishing semiconductor wafers and, in particular, to floating subcarriers therefor.
2. Description of the Prior Art
Currently, silicon wafers for semiconductors are polished with machines using rigid metallic multi-wafer support chucks. Exemplary of the prior art are the apparatuses disclosed in U.S. Pat. No. 4,194,324 issued on Mar. 25, 1980, to Bonora et al. and U.S. Pat. No. 4,132,037 issued on Jan. 2, 1979, to Bonora, both of which are owned by the assignee of the present invention.
In a conventional template or so-called "insert process", the wafers are mounted in pockets on the surface of a chuck or subcarrier. The planes in the pockets in which the wafers are situated are all parallel and, more importantly, are all at the same elevation. This configuration gives rise to some problems. First, the wafers must be sorted before polishing so that all wafers being polished at a given time are approximately the same thickness. The typical range of thicknesses for wafers that are polished during any one batch process is approximately 0.0002 inches. This degree of sorting is required because the conventional so-called "one-plane polishing technique" is not able to adapt to individual thicknesses that are very different. If there is no preliminary sorting, taper can occur in the finished wafers due to uneven treatment during polishing of individual wafers with different thicknesses.
The present invention permits the adaptation of rigid chucks for improved multi-wafer polishing. Each wafer support chuck is retained in a multi-chuck carrier assembly with the vertical polishing force applied to the subcarriers by either air pressure or liquid pressure or mechanical force. Thus, each subcarrier is free to accommodate the wafers of varying thicknesses and tapers.
When air or liquid pressure is used, an elastomeric O-ring provides both a sealing function and a support function for the lateral force developed by polishing. Air or fluid pressure applied to a sealed chamber behind the subcarrier assemblies creates a downwardly directed force for polishing while the wafers are frictionally retained on the bottom side of each subcarrier. The individual subcarriers can be readily removed for surface conditioning or the whole chuck assembly can be lapped in place on a conventional lapping machine. Driven or non-driven rotation of each subcarrier on its own axis is also feasible.
On the other hand, another feature of this invention is that there may be free rotation of the floating subcarrier. This extra rotational freedom of each floating subcarrier gives the desired improved tight-tolerance flatness of the wafer. This feature is accomplished in one basic variation by using a mechanical floating subcarrier which uses a spring or other resilient means to supply a load to the wafer. This mechanical floating subcarrier is also used with spacing between the wafer edge and the associated subcarrier side wall to provide for free rotation of the wafer. However, in the variations in which there is an applied hydraulic or pneumatic force, a resilient device is interposed in the side wall in order to seal in the hydraulic fluid or the compressed air, respectively.
There are several advantages to using the present invention. First of all, production costs are lowered while wafer flatness is improved. These advantages are accomplished by allowing the individual subcarriers and, hence, the wafers to move freely during polishing and to adjust the polishing plane of the wafer faces.
Another advantage of the present invention over conventional batch-process polishers is that cycle time may be reduced and the wafer TIR may be decreased to previously unattainable levels. TIR refers to the "total indicated reading" of the maximum deviation on the wafer surface between the high point and the low point in the plane of the wafer.
Another advantage of the present invention is that consistent results are yielded and wafer thickness sorting is not required.
The accomplishment of these and other advantages will become more readily apparent from the following description of the drawings and the related discussion of the preferred embodiments.